Most Arcane builds in Elden Ring revolve around effects like Hemorrhage and Frostbite. The often overlooked Scarlet Rot builds made a comeback in Shadow of the Erdtree with the Poleblade of the Bud. After you defeat Romina for this weapon, you can build around it.
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Scarlet Rot builds are generally rare in Elden Ring because they don’t do as much damage as bleed builds or have as much crowd control as frost builds. However, with the new Poleblade of the Bud Remembrance weapon in Shadow of the Erdtree, this build will amplify your Rot damage, making it great against bosses and in PvP.
Since you're going to be causing as much Scarlet Rot as possible, this build will be an Arcane build. If you're using a build other than the base game, you'll need to respect your character before starting this build.
The most important stat in this build is Arcane, so invest 40 points into it before moving on to Vigor and Endurance. Get Vigor to 40 and Endurance to 30, then invest some points into Strength and Dexterity for scaling purposes. The rest of your remaining points are best invested into Arcane until it hits the 60 soft cap.